In the offshore production of oil and natural gas, floating vessels are frequently used to temporarily store the hydrocarbons prior to shipment to onshore production and refining facilities. The hydrocarbons are produced from an offshore structure which is usually anchored to the seafloor with piling or from a subsea production system (SPS). In both cases, the hydrocarbons are shipped, usually via a pipeline, to an offshore floating terminal which is occasionally referred to as a single-anchor-leg-mooring (SALM) system. A typical SALM is comprised of a base structure which is positioned on the marine bottom and a riser which is connected to the base structure and extends above the water surface. At the upper end of the riser is attached an articulated mooring arm which in turn is connected to a floating storage vessel. Usually, the floating vessel is a converted tanker and is permanently moored to the riser via the articulated mooring arm. The pipeline, which carries the hydrocarbons from the offshore structure or SPS, continues up along the riser and across the articulated mooring arm into the storage compartments of the floating vessel. Shuttle tankers then offload the hydrocarbons from the floating vessel for transportation to onshore facilities.
In an offshore environment the articulated mooring arm must be sufficiently flexible to accommodate the movement of the vessel relative to the riser. As the vessel is acted upon by forces induced by winds, waves, ice masses, ocean currents, etc., the vessel will roll, pitch and heave. In addition, the vessel will yaw about its mooring point as the direction of the forces vary.
During an emergency condition such as the arrival of a hurricane or ice masses, it may be necessary to effect a quick disconnection of the vessel from the riser. The emergency situation is aggravated by the fact that the disconnection must occur quickly enough to prevent damage to the vessel, articulated mooring arm, and riser. Once disconnected, the movement of the vessel forward into the articulated mooring arm as a result of sea movement can cause significant damage to the vessel, articulated mooring arm, and riser.
Accordingly, the need exists for an improved apparatus which effects the quick and accurately-timed disconnection of a floating storage vessel from the riser of an SALM in a manner which permits an operator time to safely reverse the vessel away from the riser before any contact is made.